The present invention generally resides in the art of rheometer systems for testing polymers. More particularly, the present invention relates to a variable eccentric cam that is incorporated into to a rheometer system for converting rotary motion at a motor-driven drive shaft into oscillatory motion at a polymer sample die.
Polymers are most often tested according to one of five ASTM methods, namely, ASTM D1646, D2084, D5289, D6204 and D6601. Method D1646 describes the use of a shearing disk viscometer to measure the viscosity and scorch characteristics of a polymer. In this method a rotor continuously rotates, as opposed to oscillating. The second method, D2084, describes a curemeter utilizing an oscillating rotor. The degree of oscillation is fixed and determines the percent strain, such that the device employed is a constant strain instrument. Method D 5289 describes three rotorless curemeter systems wherein one die is oscillated at a fixed amplitude. As in the methods previously discussed, this amplitude is fixed, and the instrument is, therefore, a fixed strain instrument. The present invention is concerned with oscillating instruments, and may be applied to oscillating test instruments currently known or to oscillating test instruments developed in the future, when the test instrument is suitable for the incorporation of the teachings herein.
Several patents describe instruments operating in accordance with ASTM D2084 and D5289. U.S. Pat. No. 3,681,980 illustrates the application of a fixed eccentric cam to facilitate oscillation of a rotor. This amplitude of oscillation is determined by the position of the pin on the eccentric. U.S. Pat. No. 4,794,788 also describes the use of an eccentric to facilitate oscillatory motion. The amplitude of oscillation for these instruments has been fixed for any given test, although the amplitude of oscillation can be changed between tests by changing the position of the pin on the eccentric or by changing the eccentric to one with a different off-set. The structure of such an eccentric cam is shown in FIGS. 1 and 2.
In FIG. 1, it can be seen how the operation of a rotating eccentric cam 100 can convert rotary motion (represented by arrow R) at eccentric cam 100 into oscillatory motion (represented by arrow O) at a polymer sample die 102. A pin 104 extends from eccentric cam 100 to connect to a first end of an eccentric arm 106. The second end of eccentric arm 106 attaches to the first end of the drive plate 108, which has its second end rigidly fixed to die shaft 110. Pin 104 is placed off center on eccentric cam 100 such that, as eccentric cam 100 is rotated by a drive shaft and motor (not shown) pin 104 rotates about the central axis of rotation of eccentric cam 100. It will be appreciated that this causes eccentric arm 106 to be alternatively pushed and pulled by the rotation of eccentric cam 100, such that drive plate 108 is also pushed in and pulled to create oscillation at die shaft 110 and die 102.
In order to achieve different amplitudes of oscillation, eccentric cam 100 might be designed as shown in FIG. 2, wherein eccentric cam 100 provides for three pin positions 104A, 104B and 104C. Each pin position 104A, B, C represents a different amplitude of oscillation, and, thus, the amplitude of oscillation may be changed between tests by changing the position of eccentric arm 106 on the eccentric cam 100 (i.e., by moving the connection of eccentric arm 106 to a new position on eccentric cam 100). While FIG. 2 shows one eccentric cam with three pin positions 104A, B, C, it should be appreciated that, rather than providing multiple pin positions on one eccentric cam 100, multiple eccentric cams 100 might be provided, each with a single pin position 104 placed at a different distance from the center axis of rotation of the eccentric cam 100. In such a case, the eccentric cam would need to be changed between tests in order to provide for differing amplitudes of oscillation.
ASTM D6204 describes the use of a constant strain test, and also discloses the capability of performing a variable strain test. ASTM D6601 fully describes the conditions for evaluating a specimen at more than one strain amplitude during a single test. In this method, a specimen is subjected to strains of 1, 2, 5, 10 and 20% during the same test. In order to use the previous apparatus as shown in FIG. 1, the test must be momentarily stopped while the strain amplitude is manually changed by either (1) changing the position of the eccentric arm on one eccentric cam (when the eccentric provides multiple pin position, as in FIG. 2), or (2) changing the eccentric cam itself (when multiple eccentrics are available having only one pin position).
In the case of (1) above, it would be difficult, if not impossible to have one eccentric with the eccentric positions necessary to achieve 1, 2, 5, 10 and 20% strain. In order to obtain these strain amplitudes for a typical rheometer geometry, the pin positions on the eccentric cam would need to be located at approximately 0.004, 0.009, 0.021, 0.043 and 0.086 inches from the center axis of rotation of the eccentric cam. Even using an eccentric pin as small as 0.125 inches, it would be nearly impossible to accurately machine these positions on one eccentric cam, along a single radius (as in FIG. 2). Even if the positions were machined on the eccentric such that they did not align along a single radius, they would have to be very accurately placed, and some means would need to be provided for distinguishing between each position.
In example (2) above, exchanging eccentric cams to obtain different strains also makes it difficult to achieve all of the strains described in ASTM D6601. In this situation, the test would have to be momentarily stopped while the eccentric cam has changed. However, the procedures require that individual eccentric cams be calibrated, and such calibration would be tedious and time consuming, since five different eccentric cams would be needed. Calibration is critical since the position of the eccentric pin determines where the amplitude crosses through zero amplitude.
To avoid the problems associated with either changing eccentric pin locations or changing eccentric cams, the current apparatus to test according to ASTM D6204 and D6601 are direct drive instruments as described in U.S. Pat. No. 5,079,956. The direct drive system is illustrated in FIG. 3. Therein, a motor 200 is programmed to oscillate, and it is directly fixed to polymer sample die 202 through a die shaft 204, such that the oscillatory motion of motor 200 is directly transferred to die 202. While these direct drive systems are easily calibrated, and allow for fast and automated changing of the oscillation amplitude, they suffer from the fact that continuously testing at small amplitudes can cause excessive wear on the motor and premature failure. In addition, the motor must be very precise in order to provide control at small amplitudes.
Thus, while the prior art has provided some means for converting rotary motion into differing amplitudes of oscillation, there exists a need in the art for a variable eccentric cam that can provide for virtually any desired amplitude of oscillation without necessitating that eccentric cams be changed or requiring that discrete positions for the attachment of the eccentric arm to the eccentric cam be chosen.
In general, the present invention provides an eccentric cam for use in oscillating rheometer systems wherein the eccentric cam is mounted on a drive shaft and attaches to an eccentric arm for converting rotary motion of the drive shaft into oscillatory motion at a rubber specimen die, which is operatively connected to the eccentric arm through a drive arm. The eccentric cam includes a slide housing that is operatively connected to the drive shaft to rotate therewith such that the slide housing has a center axis of rotation. A slide screw is mounted in the slide housing, and a slide is threaded on the slide screw. The slide includes a connector for attachment to the eccentric arm, and a means is provided for rotating the slide screw to move the slide and thereby change the position of the connector relative to the center axis of rotation of the slide housing. In one particular embodiment, the means for rotating the slide screw is an adjustment nut for manually turning the slide screw to move the slide threaded thereon. In another embodiment, the means for rotating the slide screw includes a screw actuating bevel gear and stationary bevel gear that selectively engages the screw actuating bevel gear.
The eccentric cam is to be employed in a rheometer system, such that the present invention further provides a variable eccentric rheometer system that includes a rotary drive shaft; a slide housing operatively connected to the rotary drive shaft to rotate therewith, such that the slide housing has a center axis of rotation; a slide screw mounted in the slide housing; a slide threaded on the slide screw and including a connecter; means for rotating the slide screw to move the slide and thereby change the position of the connector relative to the center axis of rotation of the slide housing; and eccentric arm having a first end and a second end, wherein the first end of the eccentric arm is fixed to the connector of the slide by a bearing; a drive plate having a first end and second end, wherein the first end of the drive plate is fixed to the second end of the eccentric of the eccentric arm; a die shaft having a first end and a second end, wherein the first end of the die shaft is rigidly fixed to the second tend of the drive plate; and a die rigidly fixed to the second end of the die shaft.
As mentioned, rotation of the slide screw causes the slide to move, which, in turn, changes the position of the connector relative to the center axis of rotation of slide housing. It is the relation of the connector to the center axis of rotation that dictates the amplitude of oscillation, and, thus, the present invention provides an eccentric cam for use in a rheometer system that is continuously variable with respect to the amplitudes of oscillation that it can bring about. In less preferred embodiments, the means of rotating the slide screw is manual, while, in more preferred embodiments, an automated means is provided for accurately achieving the desired amplitude of oscillation at the die.